MongoDB Repositories

This chapter points out the specialties for repository support for MongoDB. This chapter builds on the core repository support explained in core concepts. You should have a sound understanding of the basic concepts explained there.

Usage

To access domain entities stored in a MongoDB, you can use our sophisticated repository support that eases implementation quite significantly. To do so, create an interface for your repository, as the following example shows:

Example 1. Sample Person entity

public class Person {

  @Id
  private String id;
  private String firstname;
  private String lastname;
  private Address address;

  // … getters and setters omitted
}

Note that the domain type shown in the preceding example has a property named id of type String.The default serialization mechanism used in MongoTemplate (which backs the repository support) regards properties named id as the document ID. Currently, we support String, ObjectId, and BigInteger as ID types. Please see ID mapping for more information about on how the id field is handled in the mapping layer.

Now that we have a domain object, we can define an interface that uses it, as follows:

Basic repository interface to persist Person entities

Imperative
Reactive

public interface PersonRepository extends PagingAndSortingRepository<Person, String> {

    // additional custom query methods go here
}

public interface PersonRepository extends ReactiveSortingRepository<Person, String> {

    // additional custom query methods go here
}

To start using the repository, use the @EnableMongoRepositories annotation. That annotation carries the same attributes as the namespace element. If no base package is configured, the infrastructure scans the package of the annotated configuration class. The following example shows how to configuration your application to use MongoDB repositories:

Imperative
Reactive
XML

@Configuration
@EnableMongoRepositories("com.acme..repositories")
class ApplicationConfig extends AbstractMongoClientConfiguration {

  @Override
  protected String getDatabaseName() {
    return "e-store";
  }

  @Override
  protected String getMappingBasePackage() {
    return "com.acme..repositories";
  }
}

@Configuration
@EnableReactiveMongoRepositories("com.acme..repositories")
class ApplicationConfig extends AbstractReactiveMongoConfiguration {

  @Override
  protected String getDatabaseName() {
    return "e-store";
  }

  @Override
  protected String getMappingBasePackage() {
    return "com.acme..repositories";
  }
}

MongoDB uses two different drivers for imperative (synchronous/blocking) and reactive (non-blocking) data access. You must create a connection by using the Reactive Streams driver to provide the required infrastructure for Spring Data’s Reactive MongoDB support. Consequently, you must provide a separate configuration for MongoDB’s Reactive Streams driver. Note that your application operates on two different connections if you use reactive and blocking Spring Data MongoDB templates and repositories.

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xmlns:mongo="http://www.springframework.org/schema/data/mongo"
  xsi:schemaLocation="http://www.springframework.org/schema/beans
    https://www.springframework.org/schema/beans/spring-beans-3.0.xsd
    http://www.springframework.org/schema/data/mongo
    https://www.springframework.org/schema/data/mongo/spring-mongo-1.0.xsd">

  <mongo:mongo-client id="mongoClient" />

  <bean id="mongoTemplate" class="org.springframework.data.mongodb.core.MongoTemplate">
    <constructor-arg ref="mongoClient" />
    <constructor-arg value="databaseName" />
  </bean>

  <mongo:repositories base-package="com.acme.*.repositories" />

</beans>

This namespace element causes the base packages to be scanned for interfaces that extend MongoRepository and create Spring beans for each one found. By default, the repositories get a MongoTemplate Spring bean wired that is called mongoTemplate, so you only need to configure mongo-template-ref explicitly if you deviate from this convention.

Because our domain repository extends PagingAndSortingRepository, it provides you with methods for paginated and sorted access to the entities. In the case of reactive repositories only ReactiveSortingRepository is available since the notion of a Page is not applicable. However finder methods still accept a Sort and Limit parameter.

The reactive space offers various reactive composition libraries. The most common libraries are RxJava and Project Reactor.

Spring Data MongoDB is built on top of the MongoDB Reactive Streams driver, to provide maximal interoperability by relying on the Reactive Streams initiative. Static APIs, such as ReactiveMongoOperations, are provided by using Project Reactor’s Flux and Mono types. Project Reactor offers various adapters to convert reactive wrapper types (Flux to Observable and vice versa), but conversion can easily clutter your code.

Spring Data’s Reactive Repository abstraction is a dynamic API, mostly defined by you and your requirements as you declare query methods. Reactive MongoDB repositories can be implemented by using either RxJava or Project Reactor wrapper types by extending from one of the following library-specific repository interfaces:

ReactiveCrudRepository
ReactiveSortingRepository
RxJava3CrudRepository
RxJava3SortingRepository

Spring Data converts reactive wrapper types behind the scenes so that you can stick to your favorite composition library.

In case you want to obtain methods for basic CRUD operations also add the CrudRepository interface. Working with the repository instance is just a matter of dependency injecting it into a client . Consequently, accessing the second page of Person objects at a page size of 10 would resemble the following code:

Paging access to Person entities

Imperative
Reactive

@ExtendWith(SpringExtension.class)
@ContextConfiguration
class PersonRepositoryTests {

    @Autowired PersonRepository repository;

    @Test
    void readsFirstPageCorrectly() {

      Page<Person> persons = repository.findAll(PageRequest.of(0, 10));
      assertThat(persons.isFirstPage()).isTrue();
    }
}

@ExtendWith(SpringExtension.class)
@ContextConfiguration
class PersonRepositoryTests {

    @Autowired PersonRepository repository;

    @Test
    void readsFirstPageCorrectly() {

        Flux<Person> persons = repository.findAll(Sort.unsorted(), Limit.of(10));

        persons.as(StepVerifer::create)
            .expectNextCount(10)
            .verifyComplete();
    }
}

The preceding example creates an application context with Spring’s unit test support, which performs annotation-based dependency injection into test cases. Inside the test method, we use the repository to query the datastore. We hand the repository a PageRequest instance that requests the first page of Person objects at a page size of 10.

Type-safe Query Methods

MongoDB repository and its reactive counterpart integrates with the Querydsl project, which provides a way to perform type-safe queries.

Instead of writing queries as inline strings or externalizing them into XML files they are constructed via a fluent API.

— Querydsl Team

It provides the following features:

Code completion in the IDE (all properties, methods, and operations can be expanded in your favorite Java IDE).
Almost no syntactically invalid queries allowed (type-safe on all levels).
Domain types and properties can be referenced safely — no strings involved!
Adapts better to refactoring changes in domain types.
Incremental query definition is easier.

See the QueryDSL documentation for how to bootstrap your environment for APT-based code generation using Maven or Ant.

QueryDSL lets you write queries such as the following:

Imperative
Reactive

QPerson person = new QPerson("person");
List<Person> result = repository.findAll(person.address.zipCode.eq("C0123"));

Page<Person> page = repository.findAll(person.lastname.contains("a"),
                                       PageRequest.of(0, 2, Direction.ASC, "lastname"));

QPerson person = QPerson.person;

Flux<Person> result = repository.findAll(person.address.zipCode.eq("C0123"));

QPerson is a class that is generated by the Java annotation post-processing tool. It is a Predicate that lets you write type-safe queries. Notice that there are no strings in the query other than the C0123 value.

You can use the generated Predicate class by using the QuerydslPredicateExecutor / ReactiveQuerydslPredicateExecutor interface, which the following listing shows:

Imperative
Reactive

public interface QuerydslPredicateExecutor<T> {

    T findOne(Predicate predicate);

    List<T> findAll(Predicate predicate);

    List<T> findAll(Predicate predicate, Sort sort);

    List<T> findAll(Predicate predicate, OrderSpecifier<?>... orders);

    Page<T> findAll(Predicate predicate, Pageable pageable);

    List<T> findAll(OrderSpecifier<?>... orders);

    Long count(Predicate predicate);

    Boolean exists(Predicate predicate);
}

interface ReactiveQuerydslPredicateExecutor<T> {

    Mono<T> findOne(Predicate predicate);

    Flux<T> findAll(Predicate predicate);

    Flux<T> findAll(Predicate predicate, Sort sort);

    Flux<T> findAll(Predicate predicate, OrderSpecifier<?>... orders);

    Flux<T> findAll(OrderSpecifier<?>... orders);

    Mono<Long> count(Predicate predicate);

    Mono<Boolean> exists(Predicate predicate);
}

To use this in your repository implementation, add it to the list of repository interfaces from which your interface inherits, as the following example shows:

Imperative
Reactive

interface PersonRepository extends MongoRepository<Person, String>, QuerydslPredicateExecutor<Person> {

   // additional query methods go here
}

interface PersonRepository extends ReactiveMongoRepository<Person, String>, ReactiveQuerydslPredicateExecutor<Person> {

   // additional query methods go here
}

Please note that joins (DBRef’s) are not supported with Reactive MongoDB support.